Not true. I started with an unlocked GSM Pre 2 and did the Frankenpre conversion with my old Sprint Pre-, so I now have functional Sprint Frankepre 2.
Best directions are here:
Sprint Pre 2 - WebOS Internals
webOS world does indeed provide this service for a fee.
The best thing you can do if you want to do it yourself and are nervous: Buy a broken Pre for next to nothing and get a feel for taking it apart. It's not hard once you've grown accustomed, but it does feel quite daunting until you "get" it. It really is a "feel" thing, especially disconnecting/reconnecting the ribbon cables and sliding the side clips back in. So get one you don't mind breaking if you're worried.